Pigment grade corrosion inhibitor host-guest compositions and procedure

ABSTRACT

A pigment grade corrosion inhibitor and a method of applying the inhibitor is disclosed. The inhibitor is comprised of a host species comprised of an inorganic compound having a layered structure and a guest species comprised an anionic species of a weak acid. The host species is preferably a double hydroxide having a structure of: [M(II) 1-x  M(III) x  (OH − ) 2 ] [A n−   x/n ·y H 2 O], where M(II) is a divalent metal cationic species, M(III) is a trivalent metal cationic species, and A n−  is an anionic species, preferably with the species present in a range of: 0.2≦M(III)/(M(II)+M(III))≦0.4. The guest species include: various phosphoric acids and derivatives thereof; boric acid; cyanamidic acid; nitrous acid; derivatives of thio- and dithiocarbonic acid; thio-organic compounds functionalized with at least one —SH group of acidic character, including: 2,5-dimercapto-1,3,4-thiadiazole (DMTD), 2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4 thiadiazole, trithiocyanuric acid (TMT), and dithiocyanuric acid.

RELATED APPLICATIONS

This application is a continuation of co-pending patent application Ser.No. 10/877,946, filed 25 Jun. 2004, which claims the benefit ofProvisional Patent Application Ser. No. 60/483,436 filed 27 Jun. 2003,expired.

BACKGROUND OF THE INVENTION

Protection of metals and metal alloys against atmospheric corrosionconstitutes a challenge of significant economic importance. There aretwo distinct corrosion control technologies commonly applied to protectagainst atmospheric corrosion: conversion coatings and organic coatings.The present invention relates to novel pigment grade host-guestcompositions that are suitable for applications in organic coatings andorganic primers intended for metal protection. The guest species of thecompositions possess corrosion inhibition ability, and inhibition isachieved when the guest species are spontaneously released from the hostmatrix into aqueous environments in contact with corroding metalsubstrates.

Pigment grade corrosion inhibitors are generally employed as functionalconstituents of organic coatings or primers, with organic primersconsidered the most versatile control technology of metal corrosionunder atmospheric conditions.

Currently, when the barrier function of the primer is “lost”, corrosioninhibiting pigments provide the only known protective mechanism atdamage sites of metal supported organic coatings. This protectivemechanism implies leaching of inhibitor species from coatings while incontact with an aqueous phase and transportation of the inhibitorspecies by diffusion to specific damage sites of coatings wherecorrosion occurs. It will be apparent that pigmented primers underatmospheric corrosion conditions function like reservoirs of corrosioninhibitor species, with the reservoirs opening during aqueous events.Likewise, the effectiveness in corrosion inhibition of the primerreservoir depends on the chemical identity, solubility and load(referred to as Pigment Volume Concentration or PVC) of the inhibitorspecies.

As is well known, pigment grade corrosion inhibitors used in organicprimers must contain anionic species with effective inhibitor activityand must be characterized by limited, but effective, solubility inwater. For these reasons, Cro₄ ⁻ is the corrosion inhibitor speciespreferred in both conversion coating and high performance organic primertechnologies applied on metals for protection against atmosphericcorrosion.

It is also well known that corrosion inhibitor pigments are comprised ofselected inorganic salts, specifically inorganic salts of weakoxi-acids, or electrolytes, with limited solubility in water. It hasbeen noted, that while the anionic constituents comprise the activecorrosion inhibitor species of pigments, cationic constituents determineessential properties of the latter, such as solubility, hydrolysis pH,and specific gravity. Such is evident with the chromate series ofinhibitor pigments (where CrO₄ ⁻ is the active inhibitor), whichincludes Ca, Ba, Sr and Zn-chromates. Current research anddevelopment-activities in this field are focused on development ofeffective pigment grade corrosion inhibitors and, specifically, on thedevelopment of effective, non-toxic replacements for chromates in highperformance organic coatings, which are used as coil and aircraftprimers.

Aircraft primers and coil primers are the typical high performanceorganic coatings that are applied for protection of metals againstatmospheric corrosion, most notably for aluminum protection, andespecially in the aircraft manufacturing industry. SrCrO₄ is thecorrosion inhibitor pigment of choice for aircraft and coil primers, andis the standard in the industry. Due to environmental concerns, findinga replacement for chromates in organic coatings constitutes a mainobjective of contemporary research in this field. Likewise, efforts havebeen made to expand the application of organic corrosion inhibitors foruse in pigment grade compositions for all metals and not justspecifically aluminum.

It is generally known, that the number of inorganic anionic corrosioninhibitor species suitable for pigment synthesis and available forchromate replacement is limited essentially to a few, and specificallyto MoO₄ ⁻, PO₄ ⁻, BO₂ ⁻, SiO₄ ⁻ and NCN⁻. As a consequence, allcommercial non-chromate corrosion inhibitor pigments are molybdates,phosphates, borates, silicates or cyanamides, or combinations of thesecompounds. It should be noted that, NO₂ ⁻, a very effective inhibitor,is not available in pigment grades, since all nitrites are too solublefor coating applications. Likewise, it should be noted that some anionicspecies, such as Cl⁻, SO₄ ⁻, SO₃ ⁻ and most notably, NO₃ ⁻ to someextent, are known promoters of metal corrosion, rather than inhibitors.

In comparison to CrO₄ ⁻, inherent limitations of their corrosionpreventing mechanism render these above-specified species less effectiveinhibitors of corrosion, in general. Consequently, it appears thatinorganic chemistry is unable to produce an effective, non-toxicalternative of CrO₄ ⁻. In contrast, a large arsenal of organic corrosioninhibitor is known and applied in various corrosion controltechnologies. Mechanistic shortcomings, excessive solubility in waterand/or volatility of most of the known organic inhibitors appear to bethe physical properties inconsistent with applications in organiccoatings. Thus, requirements for pigment grade inhibitors, delimited bythe above suggested reservoir model of pigmented organic coatings,include, among other requirements, a solid consistency, non-volatility,a limited, though effective, solubility in water, high load of theinhibitor species, an efficient inhibitor mechanism, and further havinga compatible environmental profile.

An area of interest in this regard has been the development ofhydrotalcite based products for corrosion inhibition. As discussed laterin more details, hydrotalcite belongs to the family of mixed hydroxidesof layered structure, which possess anion-exchange capability and areknown to form a considerable number of derivatives containing diverseguest anions.

Hydrotalcite has found various applications, such as Cl⁻ scavenger inplastics and as an acid neutralizer in various systems.

Specifically referring to applications in corrosion inhibition, forinstance, Buchheit et al., U.S. Re. No. 35,576, (U.S. Pat. No.5,266,356) describes the in situ and spontaneous formation of ahydrotalcite-like coatings on aluminum alloys for their corrosionprotection. However, the patent is limited in that it does not mentionuse of inorganic or organic inhibitor anionic species in this context.Kuroda et al., U.S. Pat. No. 5,595,747, demonstrates the effectivenessof hydrotalcite, through its ion exchange property, to hold functionalorganic anions intended for various applications by slow release, suchas releasing an active pesticide. However, Kuroda only referencesbiocidally active compounds and does not suggest use with, or of,corrosion inhibitor compounds.

Miyata et al., U.S. Pat. No. 4,761,188, claims a hydrotalcite basedfiliform corrosion inhibiting composition containing a number ofinorganic and organic anionic species, such as I⁻, HCO₃ ²⁻, CO₃ ²⁻, CrO₄²⁻, ferrocyanide anion, and, respectively, salicylate and oxalate anion.It will be apparent however, that except for CrO₄ ²⁻, and I⁻, which isknown to rather promote corrosion, the rest of the inorganic and organicanionic species claimed within the patent are not credited with andgenerally are not recognized for corrosion inhibitor activity,specifically in coating applications. Consequently, these hydrotalcitederivatives cannot be considered pigment grade corrosion inhibitors.

As for organic corrosion inhibitors employed in organic coatings, itshould be noted that Sinko (U.S. Pat. No. 6,139,610) discloses theapplication of selected organic compounds in the form oforganic-inorganic hybrid corrosion inhibitor pigments intended forchromate replacement in organic coatings. However, Sinko does notmention the applicability of hydrotalcite specifically, for synthesizingpigment grade organic/inorganic ‘hybrid’ inhibitors. It can be concludedthat; as of to date, the application of organic corrosion inhibitors inorganic coatings has not reached commercial significance.

SUMMARY OF THE INVENTION

It has been discovered pursuant to the present invention that selectedorganic compounds in a host-guest composition may function as pigmentgrade corrosion inhibitor. Specifically, the host matrix is inorganic,it possesses a layered structure and displays anion exchange capability.Such matrices include double hydroxides with layered structure of

[M(II)_(1-x)M(III)_(x)(OH⁻)₂][A^(n−) _(x/n) ·yH₂O]

generic composition, where M(II) is generally a divalent metal cationicspecies or, in some cases Li (I), M (III) is a tri-valent metal cationicspecies, and A^(n−) represent diverse anionic species. In quantitativeterms, a ratio of:

0.2≦M(III)/M(II)+M(III)≦0.4

has been suggested as feasible. It will be apparent that the 2(1−x)+3Xtotal number of cationic valences in the matrix are balanced by the2+n(x/n) number of anionic valences of OH⁻ and A^(n−), where the formeris the quantitatively dominant species.

The load of A^(n−) anionic guest species is variable between the limits,as follows:

1/n≦A^(n−)/M(III)≦1

Notably, this concept is of practical interest in the context of thepresent invention, and it suggests that the OH⁻ and A^(n−) speciescompete for the limited number of cationic sites available for guestanions in the matrix. It also suggests that under specific synthesisconditions, the load of A^(n−) in the matrix can be maximized.

Structurally, in these double hydroxides, the OH⁻ and A^(n−) anionicspecies, as well as H₂O, reside in the space between parallel layersformed by the M(II) and M(III) cationic species. While the layeredstructure is preserved in the process, these double hydroxides displayanion exchange capability, which is apparently potentially unlimited inrespect of the chemical identity and structural diversity of theapplicable A^(n−) species.

Hydrotalcite is one well-documented representative of the family ofdouble hydroxides having a layered structure. Heretofore symbolized byHtlc-CO₃, it is a Mg(II)-Al(III)-OH—CO₃ ²⁻ system, and although somevariations of the Al/Mg molar ratio are possible, it can be fairlydescribed by the general formula of Mg₆Al₂ (OH)₁₆CO_(3·4)H₂O.

It is known however that a considerable number of doublehydroxide-carbonates of distinctly different chemical composition canexist, where M(II) can be:

Mg(II), Cu(II), Ni(II), Co(II), Zn(II), Fe(II), Mn(II), Cd(II), Pb(II),and apparently Ca(II), Sr(II) or mixtures of these,

and where M(III) can be:

Al(III), Ga(III), Ni(III), Co(III), Fe(III), Mn(III), Cr(III), V(III),as well as apparently Ti, In, Ce(III), La(III) or mixtures of all of theabove.

Further, as previously noted, M(II) also can be replaced by Li(I). Whilethe disclosure generally focuses on M(II) as being a divalent species,it should be understood that the scope of the invention will encompassmonovalent species having similar characteristics to the divalentspecies.

Several of the above suggested systems have been synthesized andcharacterized, such as: Mg—Al—CO₃ (hydrotalcite, or Htlc-CO₃ asdiscussed above), Mg—Fe—CO₃ (pyroaurite), Mg—Cr—CO₃, Mg—Mn—CO₃,Ni—Al—CO₃, Ni—Fe—CO₃, Zn—Al—CO₃, Cu—Zn—Al—CO₃, Mg—Zn—Al—CO₃,Cu—CO—Zn—Al—CO₃, and also Li—Al—CO₃ where Li(I) replaces M(II).

Finally, and as suggested above, it will be apparent that the diversityin composition of these double hydroxide systems with layered structuresis significantly increased by the large number of applicable anionicspecies of diverse chemical identity and structural character. Aconsiderable number of layered double hydroxides of modified chemicalcomposition are known, containing diverse guest anionic species otherthan CO₃ ²⁻.

Such guest species could possess corrosion inhibitor capability if theyconstitute the anionic species of selected weak acids. Without limitingthe scope of the present invention, and in addition to CrO₄ ²⁻ known tothe art, some examples of such weak acids credited with corrosioninhibitor quality, are:

-   -   ortho-phosphoric, pyrophosphoric, tripoly-phosphoric,        polyphosphoric acid;    -   mono- and di-alkyl or aryl-esters of ortho-phosphoric and        pyro-phosphoric acid;    -   metaphosphoric, trimeta-phosphoric, poly-metaphosphoric acid;    -   phosphorous (phosphonic) acid;    -   derivatives of phosphonic acid, such as compounds known in        industrial practice as NMPA and HEDPA;    -   alkyl and aryl esters of thio-phosphoric and dithio-phosphoric        acid;    -   molybdic, phospho-molybdic, silico-molybdic acid;    -   boric acid;    -   cyanamidic acid;    -   nitrous acid;        -   derivatives of thio- and dithiocarbonic acid, such as            o-alkyl esters        -   derivatives of dithiocarbamic acid, such as N-alkyl            dithiocarbamates;        -   pyrrolidinecarbodithioic acid;        -   various thio-organic compounds functionalized with one or            multiple —SH group of acidic character, including:

2,5-dimercapto-1,3,4-thiadiazole or Bismuthiol I, and2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4-thiadiazole or C₃H₂N₄S₃, and1,3,5-triazine-2,4,6(1H,3H,5H)-trithione, or trithiocyanuric acid (TMT),and dithiocyanuric acid,

-   -   various N,N-, S,S- and N,S-substituted derivatives of the above        compounds, such as        5-mercapto-3-phenyl-1,3,4-thiadiazoline-2-thione or Bismuthiol        II and 5,5′ thio-bis(1,3,4 thiadiazole-2(3H)-thione;    -   various S-substituted derivatives of trithiocyanuric acid;    -   dimer and polymer derivatives of the above, resulting from        oxidative dimerization or polymerization of di- and        poly-mercapto compounds, such as: 5,5′ dithio-bis(1,3,4        thiadiazole-2(3H)-thione or (DMTD)₂, and (DMTD)_(n), the polymer        of DMTD and(TMT)₂, the dimer and polymers of TMT;    -   various combinations of all of the above;    -   soluble salts of DMTD and TMT;        poly-ammonium salt of DMTD or (DMTD)_(n) and TMT formed with        polyamines;    -   selected mercapto derivatives: mercapto-benzothiazole,        mercapto-benzoxazole, mercapto-benzimidazole, or combinations of        the above    -   di- or poly-mercapto organic compounds such as:        -   di-mercapto derivatives of thiophene, pyrrole, furane, and            of diazoles and thiadiazoles;        -   di- and tri-mercapto derivatives of pyridine, diazines,            triazines and of benzimidazole and benzothiazole, such as:    -   dimercaptopyridine, 2,4-dithiohydantoine, and        2,4-dimercapto-6-amino-5-triazine;    -   carboxylic and di-carboxylic acids such as: ascorbic, salicylic        acid, phthalic acid, nitro-phthalic acid and succinic acid; and        derivatives of succinic acid such as:        1-(benzothiazol-2-ylthio)succinic acid.

It will be apparent that, with no intent to limit the scope of thepresent invention, the above described host-guest compositions can alsobe employed as constituents of ordinary physical mixtures with variouscorrosion inhibitor pigments, preferably constituted of non-toxiccationic and anionic species with corrosion inhibitor properties, suchas: Mg, Ca, Sr, La, Ce, Zn, Fe, Al, Bi and, respectively, MoO₄ ⁻, PO₄ ⁻,HPO₃ ⁻, poly-phosphates, BO₂ ⁻, SiO₄ ⁻, NCN⁻, WO₄ ⁻, phosphomolybdate,phosphotungstate, and various combinations of all of the above. Specificcompounds include: zinc phosphate, cerium molybdate, calcium silicate,strontium borate, zinc cyanamide, cerium phosphotungstate andrespectively, ZnO, CeO₂, ZrO₂ amorphous SiO₂ or combinations of thesecompounds.

Additionally, it will be evident that host-guest compositions can bealso combined, in the form of ordinary physical mixtures, with diverseconductive polymer derivatives or composites with corrosion inhibitorproperties, such as mixtures or compositions based on polyaniline,polypyrrole or polythiophene. Likewise, it is understood that the abovelist is not conclusive, and similar compounds and derivatives will yieldsimilar results.

Of the above guest species, it has been discovered that2,5-dimercapto-1,3,4 thiadiazole symbolized by HS—CN₂SC—SH or “DMTD” andits derivatives inhibit atmospheric corrosion of aluminum, including Al2024 T-3. It has been also proven that DMTD and various of itsderivatives in pigment grade form are applicable as components oforganic primers or in soluble or partially soluble form as an inhibitorconstituent of conversion coating compositions intended for aluminumprotection.

This discovery was not expected, considering that DMTD does not formessentially insoluble compounds with Al (III), of which thischaracteristic is generally a prerequisite for corrosion inhibitionactivity of organic compounds on metal substrates.

Although unexpected, this effect is explicable in light of the presentresearch, however, considering the high chemical affinity displayed byorganic thiol derivatives, in general, and specifically by DMTD and TMT,toward Cu(II) and Cu-rich surfaces. In the specific case of DMTD, it hasbeen shown that DMTD spontaneously forms stable chemisorption layers oncathodically polarized Cu surfaces and, consequently, inhibits cathodicO₂ reduction in aqueous conditions. Based on this, it can be reasonablyassumed that DMTD operates by mechanism similar to CrO₄ ⁽²⁻⁾ on(cathodic) Cu-rich intermetallics of Al-2024 in atmospheric conditions.

Along with DMTD, it has also been discovered pursuant to the presentinvention, that trithiocyanuric acid, or TMT, which can be classified asa tri-mercapto derivative, and its derivatives are also effectivecorrosion inhibitors of aluminum in a similar fashion as DMTD. It hasalso been discovered that DMTD and TMT and their derivatives areeffective corrosion inhibitors of galvanized steel and similar metalsubstrates, where these compounds interact with and protect thesacrificial zinc layer and, thus, indirectly protect the steelsubstrate.

As mentioned above, if double hydroxide systems with layered structurecontain selected guest anionic species characterized by corrosioninhibitor activity, they could function as corrosion inhibitor pigmentsin organic coating applications. It will be readily apparent however,that as high as possible of a load (in the inorganic matrix) of theinhibitor guest anionic species is required in this application.

It appears that the Htlc-CrO₄ derivative is the only double hydroxidehaving a layered structure, containing guest anionic speciescharacterized by recognized corrosion inhibitor ability, which has beensuggested by the art as corrosion inhibitor pigment. In addition to therecognized toxicity however, this known Htlc-CrO₄ derivative is alsocharacterized by a low load of the CrO₄ ⁻ inhibitor guest species, whichconstitutes a significant shortcoming of any pigment grade inhibitorintended for organic coating applications.

A feasible explanation of this shortcoming relates to the generallypracticed synthesis process of Htlc derivatives, which comprises thefollowing steps:

-   -   thermal decomposition of Htlc-CO₃ at 500-600° C. (dehydration        and decarbonation takes place resulting in a mixed oxide        precursor); and    -   reconstruction of the Htlc-like structure by exposure of the        mixed oxide precursor to aqueous solutions of various soluble        salts.

In this process, OH⁻ (results in alkaline hydrolysis) and the availableanionic species of the employed salts are absorbed and incorporated intothe reconstructed solid matrix and a Htlc-like structure forms, wherethe CO₃ ⁻ anion, which is present in the original hydrotalcite matrix,is substituted by guest anions.

However, alkaline hydrolysis of most neutral or basic salts of weakacids (such as Na₂CrO₄) affords relatively high concentration of OH⁻ inaqueous solution, which competes (in the present case with the CrO₄ ⁻species) for the limited number of sites available for the guest speciesin the matrix, resulting in low absorption, or load of CrO₄ ⁻.

As a direct consequence, the known synthesis procedure of“reconstruction” carried out by exposure of the mixed oxide precursor tosoluble salt solutions, such as for example Na₂CrO₄, in general resultsin Htlc derivatives characterized by a relatively low load of the guestspecies. As demonstrated in the following Comparative Example 2, the lowload is a significant limitation on this considered procedure, asrelated to corrosion inhibitor pigment synthesis. Due to thislimitation, Htlc derivatives produced in solutions of salts aretypically characterized by relatively low loads of the guest anionicspecies. A low content in the guest species implies a low storedinhibitor capacity, which results in a low release rate of the former,and ultimately renders such Htlc derivatives useless as corrosioninhibitor pigments.

It will be noted again, that corrosion inhibition “in degree” depends onthe available concentration of the (guest) inhibitor species in theaqueous phase of the corrosion system; that is, there is a minimumcritical concentration of the inhibitor species necessary at a corrosionsite for effective performance. It will be apparent that the availableconcentration of the inhibitor species is a direct function of the loadof the same in the pigment matrix. Consequently, it is reasonable tostate that Htlc derivatives containing non-inhibitor guest species orcarrying a low load of inhibitor species will not function as corrosioninhibitor pigments in organic coating applications, as disclosed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-9 are graphical prints representing IR spectra of productsproduced pursuant to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Although the disclosure hereof is detailed and exact to enable thoseskilled in the art to practice the invention, the physical embodimentsherein disclosed merely exemplify the invention which may be embodied inother specific structure. While the preferred embodiment has beendescribed, the details may be changed without departing from theinvention, which is defined by the claims.

It will be obvious to one informed in the relevant art however, that theconcept of the present invention applies generally to all doublehydroxide systems having a layered structure and possessing anionexchange capability, as earlier specified. Specifically, however, andwith no intent on limiting the scope of the present invention, thesubsequent disclosures which relate to the synthesis of pigment gradecorrosion inhibitor host-guest compositions generally will beexemplified by hydrotalcite based systems, that is, by the synthesis ofdouble hydroxides with layered structure of:

[M(II)_(1-x)M(III)_(x)(OH⁻)₂][A^(n−) _(x/n) ·yH₂O],

stated as a generic composition, where M(II) and M(III) are Mg(II) andAl(III), respectively, and A^(n−) is diverse corrosion inhibitor anionicspecies.

Htlc matrices and related Mg—Al—O mixed oxide precursors, employed inthe reconstruction procedure of Htlc derivatives, are inherently quitealkaline, having a hydrolysis pH˜11, observed pursuant to the presentinvention. If the reconstruction synthesis is performed in alkaline saltsolutions, this inherent alkalinity additionally limits the absorptionof guest anionic species into Htlc matrices, resulting in Htlcderivatives of low load in guest species. As indicated earlier, the loadof A^(n−) guest anionic species is variable between the limits of1/n≦A^(n−)/M(III)≦1 in Htlc derivatives and potentially will beminimized by OH—, the dominant anionic species in these systems.

It has been discovered, pursuant to the present invention, that thereconstruction procedure performed in solutions or dispersions of weakacids, acid salts of weak poly-basic acids or neutral salts withhydrolysis pH<9, rather than in solutions of alkaline salts of suchacids, yields a high load of the guest anionic species in the resultingHtlc derivatives. It is the current understanding that because of thecomparatively low concentration of OH⁻ in these systems, the competitionfor the limited number of sites available for guest species in thematrix tends to maximize the absorption of the guest anionic species.This maximized absorption is apparent primarily with acids which do notform insoluble salts with Mg(II) and/or Al(III) cations, such asthio-organic compounds with acidic —SH functionalities, as demonstratedin several subsequently presented Examples. (contrast ComparativeExample 2 with Examples 2-4).

It has been also discovered pursuant to the present invention thatabsorption of guest species of neutral or alkaline salts can be enhancedand assisted by employing strong acids, preferably mono-basic acids forpH adjustment. For example, HNO₃ can be employed to enhance absorptionof anions of alkaline salts into an Htlc matrix. It has been furtherobserved that poly-valent anionic species appear to display enhancedaffinity toward Htlc matrices. Poly-valent species are preferablyabsorbed as compared to mono-valent species.

Also pursuant to the present invention, several new Htlc derivativeshave been synthesized, containing guest anionic species with corrosioninhibitor ability, as follows: MoO₄ ⁻, PO₄ ⁻, BO₂ ⁻, NCN⁻, NO₂ ⁻, and2,5-dimercapto-1,3,4-thiadiazole⁽⁻⁾ (“DMTD”) 2-mercaptobenzothiazole⁽⁻⁾,and trithiocyanuric acid⁽⁻⁾ (TMT). Htlc derivatives containing CrO₄ ⁻species, which are known to the art, were also synthesized, but with asignificantly higher guest species content, as described in Example 2.

These pigment grade products are generally characterized by high guestanionic species content and are applicable in paint formulations andorganic coatings. Once in contact with an aqueous medium, it has beenobserved that the Htlc derivatives produced according to the presentinvention release guest anionic species by dissolution and by anionexchange at rates controlled by the environment. Thus, these derivativesfunction as pigment grade corrosion inhibitors, thereby greatlyextending the number of known corrosion inhibitor pigments.

In practice, the reconstruction synthesis was carried out by dispersinga finely ground Mg—Al—O mixed oxide precursor (obtained by thermaldecomposition of Htlc-CO₃) in a solution or dispersion of weak acids,anhydrides or acid salts or neutral salts of weak acids. The mixture wasthen extensively stirred. Notably, in the case of some weak acids, suchas phosphoric acid, which form insoluble phosphates with Mg(II) andAl(III), the conversion of the solid matrix was prevented by maintaininga pH>8 in the system. Subsequent to the completion of the absorptionprocess, the solid phases were separated by filtration. The resultantpresscakes were than washed extensively with a volume of waterapproximately 2-5 times the volume of the filtrate, dried overnight at110° C., and pulverized. The guest species load of the Htlc derivativeswas determined by analysis of the resultant filtrates and wash watersfor contents of the same species. In some specific cases, as noted inthe following relevant examples, the IR spectrum of the synthesizedpigment grade Htlc derivative has an altered structure in comparison toHtlc and indicates the presence of the guest species.

Comparative Examples

The examples subsequently presented are intended to provide evidenceregarding existing limitations of the art in comparison to the presentinvention. More specifically, the subsequent examples are intended topresent the “reconstruction” procedure currently practiced in the artand to identify the only known, prior to this invention, Htlc derivativecontaining guest anionic species with corrosion inhibitor capability,which is Htlc-CrO₄ ²⁻.

Comparative Example 1

A generic Mg—Al—CO₃—OH type of hydrotalcite, Htlc-CO₃ containing about9% CO₃, (from Sigma-Aldrich), was thermally decomposed by ignition asknown in the art, as follows: approximately 50 g of finely ground suchproduct was heated to approximately 600° C., maintained for about one(1) hour, and subsequently cooled in a closed environment. A weight lossof approximately 43% was observed, due to decarbonation and dehydration.The resulting solid powder, believed to be a mixed Mg—Al—O oxide, withan appearance similar to the original Htlc, was used in all subsequentexamples of the present invention. Relevant analytical data arepresented below in Table 1. FIG. 1 presents both, the IR spectrum of theoriginal non-ignited Htlc and of the mixed Mg—Al—O oxide.

TABLE 1 Appearance White powder Yield (g) 28.5 CO₃ % 0 pH, in saturatedleachet 10.7

Comparative Example 2

A generic Mg—Al—OH—CrO₄ ²⁻ type of Htlc was produced by thereconstruction procedure known to the art as follows: 20 g of finelyground mixed Mg—Al—O oxide precursor (see Comparative Example 1), wasdispersed into 200 ml of a 0.2M Na₂CrO₄ solution, stirred intensively atambient temperature for four (4) hours, and filtered. The resultantpresscake was washed extensively with a volume of water approximatelyfour times that of the volume of the filtrate and subsequently driedovernight at 110° C. The filtrate appeared yellow in color, and afiltrate pH=13 was observed. This is an indication that, under the givenpractical condition, the absorption capacity of the solid matrix wasexhausted and the maximum load of CrO₄ ²⁻ possible under theexperimental conditions was absorbed into the matrix of the resultantHtlc-CrO₄ ²⁻ derivative. The CrO₄ ²⁻ content of the product wasdetermined by iodometric titration of the filtrate and the wash water.Pertinent analytical data are presented below in Table 2. Thecomparatively low load of the CrO₄ ²⁻ (5.8% by weight) was noted assignificant.

TABLE 2 Htlc-CrO₄ ²⁻ Results (1) Appearance Light yellow powder Yield(g) 28.0 CrO₄ ²⁻ % 5.8 pH, in saturated leachet 9.3

Referential Examples

The following examples are references to be used in formulating specificexamples and test subjects to be used in connection with specificexamples listed in the EXAMPLE section.

Referential Example 1

This example is intended to disclose one synthesis procedure applicablefor incorporating DMTD into a complex solid matrix corresponding to thegeneral composition of 45% Zn(DMTD)₂/32% Zn₃(PO₄)₂ 2H₂O/23% ZnO.

In practice, the synthesis was carried out as follows:

6.33 moles (515.0 g) of high grade ZnO (0.25 micron average particlesize), was re-slurried in 2000 ml water at 50-60° C. and intenseagitation for 1 (one) hour. After that, 1.5 moles of H₃PO₄, as 50%solution, were introduced gradually into the ZnO slurry and the sameconditions were continued for 30 minutes. Subsequently, an aqueoussuspension of 2.5 moles of DMTD in 1500 ml water was introduced in about30 minutes. The intensively stirred slurry was heated to 75-80° C. andthe same conditions were maintained for 2 (two) hours. The solid phasewas isolated by filtration, dried at 100-105° C. to 0.5-2% moisturecontent and pulverized.

Relevant analytical data are presented below, in Table A.

TABLE A Measured quality parameters Determined values appearance Lightyellow powder specific gravity 2.7 solubility, at 24° C. 0.3 g/l pH(saturated extract) 5-6 oil abortion, lbs/100 lbs 33 yield, g 992

Referential Example 2

Pigment grade Sr-doped amorphous silica of SrSiO₃·11SiO₂·5·7H₂Ocomposition, containing approximately 9.5% Sr species, was synthesizedaccording to the following procedure:

Initially, solution A was prepared by reacting 0.51 mole of SrCO₃ and3.5 moles of HNO₃ and dissolving the composition in 1300 ml of water.Solution B was prepared by dissolving 1.9 moles of sodium silicate ofNa₂O(SiO₂)₃₂₂ composition (from Hydrite Chemical Co., WI.), in 900 ml ofwater.

Solutions A and B were delivered simultaneously and with identical ratesfor approximately 1 (one) hour into 500 ml of intensively stirred waterat 70-85° C. At the end, the pH was adjusted to 8-8.5 and the sameconditions were maintained for an additional 2 (two) hours, after whichthe resultant solid phase was separated by filtration, washed to solublesalt-free conditions, dried at approximately 105° C. overnight, andpulverized.

Relevant analytical data and IR spectrum results are presented below inTable B and FIG. 7, respectively.

TABLE B Measured Parameters Determined Values appearance White powderspecific gravity 1.8-1.9 pH(saturated extract) 9.0-9.3 oil absorption,lbs/100 lbs 52-60 Sr, % (calculated) 9.5 H₂O, % (by ignition at 600° C.)16.5 yield, g 471

Referential Example 3

A pigment grade mixture of trithiocyanuric acid+Sr-doped AmorphousSilica of SrSiO₃·11SiO₂·5H₂O+1TMT (approximate composition), containingabout 8% Sr (calculated) and 17% TMT (calculated), was produced asfollows:

100 g of trithiocyanuric acid, in powder form, were blended into 460 gof Sr-doped amorphous silica in dry granular form. The Sr-dopedamorphous silica was synthesized and processed as shown in ReferentialExample 2. The obtained mixture was subsequently pulverized to afineness of about 6 on the Hegman scale.

Trithiocyanuric acid was obtained from an aqueous solution oftri-sodium-trithiocyanurate, by adjusting the pH of the solution toabout 3, filtering, washing, and drying the resultant solid phase.

Relevant analytical data and IR spectrum results are presented below inTable C and in FIG. 8, respectively.

TABLE C Measured Parameters Determined Values appearance Light yellowpowder specific gravity 1.7 pH(saturated extract) 6.9 oil absorption,lbs/100 lbs 75-85 Sr, % (calculated) 7.9 TMT % (calculated) 17 yield, g560

Referential Example 4

This example is intended to demonstrate the application oftrithiocyanuric acid (“TMT”) as a corrosion inhibitor constituent of anamorphous silica+TMT pigment grade mixture in a typical coil coatingformulation.

The pigment grade mixture of SrSiO₃·11SiO₂·5H₂O+1TMT composition wassynthesized according to the process in Referential Example 3, and wastested (See Test formulation, Table D) on galvanized steel (from L.T.V.Steel Co.), in comparison with commercial strontium chromate (Control Aformulation, Table D), the “gold” standard of the industry for corrosioninhibitor pigments, and respectively, Sr-doped amorphous silicasynthesized according to Referential Example 2 (Control B formulation,Table D).

The typical solvent-borne polyester coil primer formulation isspecifically recommended for galvanized steel protection. Description ofthe test formulation, and control formulations A and B are presentedbelow in Table D.

TABLE D Parts by Weight Trade Names & Control Components of Suppliers ofTest Formulation Formulations Components Formulation A B Polyester ResinEPS 3302 (1) 536.0 536.0 536.0 Solvents Aromatic 150 118.0 118.0 118.0Diacetone 73.5 73.5 73.5 Alcohol Fillers RCL-535 TiO₂ (2) 46.0 46.0 46.0Aerosil R972 (3) 2.1 2.1 2.1 Catalyst Cycat 4040 (4) 7.6 7.6 7.6Hardener Cymel 303 (4) 73.6 73.6 73.6 Corrosion Inhibitor PigmentsStrontium SrCrO₄-176 (5) — 143.5 — Chromate Sr-doped As shown in — —120.0 amorphous Referential silica Example 2 Sr-doped silica + As shownin 150.0 — — TMT pigment Referential grade mixture Example 3 TotalWeight 1006.8 1000.3 976.8 Raw Material (1) Engineering PolymerSolutions Suppliers: (2) Millennium Inorganic Materials (3) DeGussaCorporation (4) Cytec. (5) Wayne Pigment Corporation

The formulation was ground to a fineness of 6.5-7.0 Hegman beforeapplication.

Referential Example 5

Di-cyclohexyl mono-ammonium salt of trithiocyanuric acid was synthesizedaccording to the following procedure

0.1 mole of di-cyclohexylamine (from Aldrich Chemical), dissolved in0.15 moles of H₂SO₄ solution of approximately 20%, was subsequentlyreacted by agitation with 0.1 mole of Na-trithiocyanurate (from AldrichChemical) dissolved in 100 ml water. After the pH was adjusted to6.5-7.0, the resulting slurry was filtered, washed to a soluble saltfree condition, dried at approximately 100° C., and the solid productwas subsequently pulverized.

Yield: 34 g, 95% of theoretical.

The relevant IR spectrum is presented in FIG. 6.

Examples

All subsequently presented Examples are intended to provide detaileddescriptions to demonstrate the effectiveness of the present inventionwith respect to corrosion inhibition.

Example 1

A generic Mg—Al—OH—CrO₄ ²⁻ type of Htlc was produced in a similarfashion as described in Comparative Example 2, except that in thepresent example, 20 g of the Mg—Al—O precursor (see ComparativeExample 1) was dispersed in 200 ml of 0.2M Na₂Cr₂O₇ (an acidic salt ofchromic acid) and stirred for (4) four hours. The filtrate appearedyellow color and had a pH=13. Pertinent analytical data is presentedbelow in Table 3.

TABLE 3 Htlc-CrO₄ ²⁻ Results (2) Appearance Yellow powder Yield (g) 33.0CrO₄ ²⁻ % 10.8

Example 2

A generic Mg—Al—OH—CrO₄ ²⁻ type of Htlc was produced in a similarfashion as described in Example 1, except that in the present example,20 g of the Mg—Al-o precursor was dispersed in 200 ml solutioncontaining 10.0 g (0.1 moles) of CrO₃ (chromic acid) and stirred for 4(four) hours. After filtration, the resulting presscake was washedthoroughly, until the wash water filtered through the presscake wascolorless. A total filtrate volume of about 800 ml was collected,appearing yellow with a pH=7.5. Pertinent analytical data is presentedbelow in Table 4. The high CrO₄ ²⁻ load (19% by weight) of the resultantHtlc derivative when compared to Comparative Example 2 was noted assignificant.

TABLE 4 Htlc-CrO₄ ²⁻ Results (3) Appearance Yellow powder Yield (g) 33.4CrO₄ ²⁻ % 19.2

Example 3

A generic Mg—Al—OH-DMTD (where DMTD stands for2,5-dimercapto-1,3,4-thiadiazole) type of Htlc was produced followingthe procedure described in Example 1, except that in the present example20 g of the Mg—Al—O precursor was dispersed in 200 ml water containing0.066 moles of Na₂-DMTD and stirred for (4) four hours. The total DMTDcontent of the filtrate and wash water was determined gravimetrically byprecipitation as Pb-DMTD. The collected yellow filtrate was alkalinewith a pH=12.5. Pertinent analytical data is presented below in Table 5.

TABLE 5 Htlc-DMTD Results Appearance Yellow powder Yield (g) 28.2 DMTD %10.0

Example 4

A generic Mg—Al—OH-DMTD type of Htlc was produced following theprocedure described in Example 3, except that in the present example 20g of the Mg—Al—O precursor was dispersed in 200 ml water containing 16 gof dispersed and partially dissolved DMTD, and stirred for (4) fourhours. The collected yellow filtrate was alkaline at pH=8.5. Relevantanalytical data is shown below in Table 6 and IR spectrum is presentedin FIG. 2. Of significance, the IR spectrum of this product appearsaltered and shows the presence of the DMTD guest in the matrix. The highload of DMTD (22% by weight) was also noted.

TABLE 6 Htlc-DMTD Results (2) Appearance Yellow powder Yield (g) 32.6DMTD % 22.0

Example 5

A generic Mg—Al—OH—MoO₄ ²⁻ type of Htlc was produced in similar fashionas described in Example 1 except that in the present example 20 g of theMg—Al—O mixed oxide precursor was dispersed in 200 ml solutioncontaining 0.04 moles of Na₂MoO₄. The total MoO₄ ²⁻ content of thefiltrate and wash water was determined gravimetrically by precipitationas SrMoO₄. The collected filtrate was alkaline at pH=13. Relevantanalytical data is presented below in Table 7.

TABLE 7 Htlc-MoO₄ Results (1) Appearance White powder Yield (g) 29.8MoO₄ % 7.9

Example 6

A generic Mg—Al—OH—MoO₄ ²⁻ type of Htlc was produced according toExample 5 except that in the present example 20 g of the Mg—Al—O mixedoxide precursor was dispersed in 200 ml water containing 8.0 g of MoO₃and was stirred for (4) four hours. The resultant filtrate's pH was 9.7.Relevant analytical data is presented below in Table 8. Thecomparatively high MoO₄ ²⁻ load of the resultant Htlc derivative wasnoted.

TABLE 8 Htlc-MoO₄ Results (2) Appearance White powder Yield (g) 37.6MoO₄ % 20.8

Example 7

A generic Mg—Al—OH-TMT type of Htlc (where TMT stands for1,3,5-Triazine-2,4,6(1H,3H,5H)-trithione, or trithiocyanuric acid wasproduced in similar fashion as described in Example 3, except that inthe present example, 20 g of the Mg—Al—O mixed oxide precursor wasdispersed in 200 ml water containing 0.02 moles of tri-sodium salt ofTMT and was stirred for (4) four hours. The collected wash water mixedwith the filtrate was analyzed for TMT content by gravimetric assessmentby precipitation at a pH=3. The resultant filtrate was very alkalinewith a pH=13.4. Relevant analytical data is presented below in Table 9.

TABLE 9 Htlc-TMT Results Appearance Off-white powder Yield (g) 29.0 TMT% 10.0

Example 8

A generic Mg—Al—OH-TMT type of Htlc was produced in similar fashion asdescribed in Example 7 except that in the present example 20 g of theMg—Al—O mixed oxide precursor was dispersed in 200 ml water containing19.0 g of TMT in dispersed form, and was stirred for (4) four hours. Theresultant filtrate had a pH=8.7. Relevant analytical data are shownbelow in Table 10 and the pertinent IR spectrum is presented in FIG. 3.The high load (40% by weight) of TMT in the resultant Htlc derivativewas noted as significant.

TABLE 10 Htlc-TMT Results (2) Appearance Off-white powder Yield (g) 40.0TMT % 40.0

Example 9

A generic Mg—Al—OH-MBT type of Htlc (where MBT stands for2-Mercaptobenzothiazole) was produced in similar fashion as described inExample 7 except that in the present example 20 g of the Mg—Al—O mixedoxide precursor was dispersed in 200 ml water containing 9.0 g of MBT indispersed form, and was stirred for (4) four hours. The resultantfiltrate was determined gravimetrically by precipitation at pH=3 tocontain approximately 0.6 g of MBT. The filtrate's pH was determined tobe 9.0. Relevant analytical data is shown below in Table 11 and relevantIR Spectrum is presented in FIG. 4. The high load (41% by weight) of MBTin the resultant Htlc derivative was noted as significant.

TABLE 11 Htlc-MBT Results Appearance Off-white powder Yield (g) 40.4 MBT% 41.6

Example 10

A generic Mg—Al—OH—BO₂ ⁻ type of Htlc was produced in similar fashion asdescribed in Example 7 except that in the present example 20 g of theMg—Al—O mixed oxide precursor was dispersed in 200 ml water containing4.44 g of boric acid (H₃BO₃) in dispersed form, and was stirred for (4)four hours. The resultant filtrate's pH was 10.3 and was found to bevoid of borate species. Pertinent analytical data is presented below inTable 12.

TABLE 12 Htlc-BO₂ ⁻ Results Appearance White powder Yield (g) 31.8 BO₂ ⁻% 9.9

Example 11

A generic Mg—Al—OH—NCN²⁻ type of Htlc was produced in similar fashion asdescribed in Example 7, except that in the present example 20 g of theMg—Al—O mixed oxide precursor was dispersed in 200 ml water containing4.5 g of H₂NCN (cyanamidic acid) and was stirred for (4) four hours. Thefiltrate's pH was 10.3 and the filtrate was found void of cyanamidespecies. Relevant analytical data is presented below in Table 13 andpertinent IR spectrum is presented in FIG. 5.

TABLE 13 Htlc-NCN²⁻ Results Appearance White powder Yield (g) 33.0 NCN²⁻% 13.6

Example 12

A generic Mg—Al—OH—NO₂ ⁻ type of Htlc was produced in similar fashion asdescribed in Example 7 except that in the present example, 20 g of theMg—Al—O mixed oxide precursor was dispersed in 0.200 ml water containing14.0 g of NaNO₂, and by gradual addition of diluted HNO₃, a pH=9 wasestablished. Subsequently, the dispersion was stirred for four hours andprocessed. The filtrate's pH was 9.7.

Analytical data and IR spectrum are presented below in Table 14 and FIG.9, respectively.

TABLE 14 Htlc-NO₂ ⁻⁻ Results Appearance White powder Yield (g) 32.0 NO₂²⁻ % Not Analyzed

Example 13

A Htlc-DMTD derivative, containing 22% DMTD, synthesized according toExample 4, was tested for DMTD release in contact with water, asfollows:

5.0 g of finely ground Htlc-DMTD was dispersed in 50 ml of water byintense stirring for 2(two) hours, after which it was filtered. Thepresscake was then washed and the resultant filtrate and wash water wereanalyzed for DMTD content. In order to assess DMTD release by anionexchange mechanism, the same process was simultaneously performed on adistinct 5.0 g of Htlc-DMTD with an additional 0.5 g of NaCl (0.008moles) introduced into the system. The amount of DMTD release was 0.06 g(0.0004 moles) when in contact with water, and 0.15 g (0.001 moles) whenin the presence of Cl⁻ species. A good correlation between Cl⁻ ionsavailable for ion exchange and released DMTD (see the above molenumbers) was observed and it was concluded that the Htlc-DMTD derivativesynthesized according to the present invention releases DMTD speciespreferentially by the anion exchange mechanism.

Example 14

A generic Mg—Al—OH—CrO₄ ²⁻ type of Htlc derivative (containing 19% CrO₄²⁻) was synthesized according to Example 2, and was tested for CrO₄ ²⁻release in water, as follows: 5.0 g of the present Htlc-CrO₄ ²⁻derivative in a finely ground form was dispersed in 100 ml water bystirring for one (1) hour. The derivative was then left to settle andthe yellow color of the supernatant was visually observed. It wasconcluded that this Htlc-CrO₄ ²⁻ derivative displays CrO₄ ²⁻ releasewhen in contact with water, similarly to the behavior of chromatepigments.

Example 15

This example shows application of one of the guest species, a DMTDderivative, as a constituent of a corrosion inhibitor pigment:

A pigment grade composite of 45% Zn(DMTD)₂/32% Zn₃(PO₄)₂·2H₂O/23% ZnO,synthesized according to Referential Example 1, was tested on aluminum,and compared to a double control: commercial strontium chromate (ControlA), which is the “gold” standard of the industry for corrosion inhibitorpigments and a molybdate-based product (Control B), which is consideredrepresentative of commercially available non-chromate corrosioninhibitor pigments. The test was performed in a typical two-componentaircraft primer formulation, specifically recommended for aluminumprotection.

The description of the different versions of this formulation, the Testprimer and of the Control A and Control B primers, are presented belowin Table 15.

TABLE 15 Trade Names & Parts by Weight Components of Suppliers ofControl Formulations Components Test A B Epoxy Base/Part A Epoxy ResinShell Epon 1001 163.0 163.0 163.0 CX75 (1) Solvents Glycol ether PM148.0 148.0 148.0 MIBK 36.7 36.7 36.7 Fillers RCL-535 TiO2 (2) 20.6 20.620.6 Min-U-Sil 15 (3) 26.0 26.0 26.0 12-50 Talc (4) 49.3 49.3 49.0Corrosion Inhibitor Pigments Zn(DMTD)₂ in See Referential 78.0 — — solidmatrix Example 1. composite (See Ref. Example 1) Strontium SrCrO4-176(5) — 107.5 — Chromate MoO₄ ⁽²⁻⁾ based Commercial (6) — — 86.0 pigment.Total part A - weight 551.0 551.0 551.0 Volume, gallons 50.0 50.0 50.0CATALYST/PART B Hardener HY-815 67.1 67.1 67.1 Polyamide (7) SolventsToluene 59.1 59.1 59.1 Isopropanol 218.5 218.5 21.5 Total Part B -weight 344.7 344.7 344.7 Volume, Gallon 50.0 50.0 50.0 Raw material (1)Shell Chemical suppliers: (2) S.C.M. Chemicals. (3) Unimin Corporation(4) Pfizer. (5) Wayne Pigment Corp. (6) The Sherwin-Williams Co. (7)Ciba-Geigy Part A (epoxy base) and Part B (catalyst) were mixed in 1:1ratio by volume, and inducted for 30 min. before application.

Example 16

This example demonstrates the efficiency of specific guest species, DMTDderivatives, in inorganic coatings in a corrosion inhibitor pigment.

In order to comparatively assess the corrosion inhibitor activity ofDMTD derivatives, the Test primer of Example 14, as well as Control Aand Control B primer formulations, were applied by wire-wound rod, onseveral Alodine 1200 (MIL-C-5541) treated bare 2024 T-3 aluminum panels(from The Q-Panel Co.), at 0.6-0.8 mils dry film thickness, aged for 7days at room temperature, scribed and subsequently subjected to saltspray exposure (according to ASTM B-117) for 2000 hours. Notably, thescribes were applied in the typical cross form, at an approximate widthof 2 mm, and, in order to remove the Alodine 1200 conversion coatingfrom the area, at an appropriate depth.

By visual examination of their physical state at the end of the testperiod, the coatings' corrosion inhibitor performance, considereddirectly proportional to the tested pigment components' corrosioninhibitive activity was qualified. The scribed area was especiallyexamined and the absence or presence of corrosion products,respectively, was interpreted as display of, or absence of, therespective corrosion inhibitor pigment's “throw power”. It will beapparent that the “throw power” is the discriminative characteristic ofeffective corrosion inhibitor pigments. Test results are summarized inTable 16.

TABLE 16 Qualification of “Throw Coating/inhibitor Performance Power”Pigment Tested Field Scribe Area Observed Test primer/Zn Intact Void ofyes (DMTD)₂ in a solid corrosion matrix(See products ReferentialExample 1) Control A/SrCr0₄ Intact Void of yes corrosion productsControl B/Mo0₄ ⁽²⁻⁾ Intact Filled with no based pigment corrosionproducts

Both Control coatings and the Test coating were found intact in thefield at the end of the test period and it was concluded that 2000 hoursof salt spray exposure was not sufficiently discriminate. Similarly toCrO₄ ⁻, DMTD displayed throw power, however, by maintaining the scribearea void of corrosion products, in a passive state for the duration ofthe salt spray exposure test. In the same conditions, MoO₄ ⁻ did notshow throw power. It was concluded that DMTD derivatives possesseffective corrosion inhibitor activity on aluminum and are applicable inpigment grades in organic primers intended for such.

Example 17

The following is an example of applicability of a guest species, DMTD insoluble forms, in conversion coatings for aluminum protection.

A DMTD based conversion coating was applied on several 2024 T-3 aluminum(the Test and Control) panels according to the following protocol:de-greasing, rinsing, deoxidizing (I), rinsing, deoxidizing (II),rinsing, treatment with DMTD (only of the Test panels), drying, posttreatment with Zr(IV)/K₂ZrF₆ solution, rinsing and drying. In practice,rinsing (performed in stirred water at ambient temperature for 1 minute)and all operations were carried out by immersion as follows:

The Test and Control panels were de-greased in an alkaline cleanersolution (containing 2% of both Na₂CO₃ and Na₃PO₄) at 50° C. for 1minute, followed by rinsing at ambient temperature for 1 minute.Deoxidizing was performed in two phases. Phase (I) was carried out in25% H₂SO₄ solution at 60° C. for 1 minute, followed by rinsing. Phase(II) was performed in 50% HNO₃ solution at ambient temperature for 30seconds, followed by subsequent rinsing. The DMTD based conversioncoating was applied (only on the Test panels) by immersion for 10minutes in saturated DMTD solution at 60° C., under agitation and,without rinsing, by subsequent drying at about 100-110° C. forapproximately 10 minutes. Both the Test and the Control panels (thelatter without DMTD coating) were post-treated by immersion, for 10minutes, in a solution containing 0.5% ZrNO₃+0.5% K₂ZrF₆, at 60° C.under agitation. The treatment was finalized by rinsing and drying theTest and Control panels at 110° C. for 10 minutes.

Example 18

In order to assess the quality of DMTD-based conversion coating on 2024T-3 aluminum, the Test panels were tested for corrosion resistance(according to ASTM B-117) and paint adhesion (tape test), in comparisonwith the Control panels, as well as with Alodine 1200 treated 2024 T-3aluminum panels, the latter being the standard of the industry. The testresults are presented below in Table 17.

TABLE 17 Corrosion resistance Rating* after 336 Paint adhesion Testedpanels hours salt spray: by tape test: Test 8, some pitting Pass Control0 Fail Standard 8, some pitting Pass *rating is considered on the 0(extensive corrosion) to 10 (no corrosion) numeric scale.

As the presented data indicates, DMTD-based conversion coating on 2024T-3 Aluminum, applied according to the present invention, possessesrobust resistance to corrosion and good paint adhesion, similar tochromate-based Alodine 1200 conversion coatings.

It was concluded that the treated DMTD derivatives are applicable ascorrosion inhibitors in conversion coating technologies intended foraluminum protection.

Example 19

This example demonstrates the applicability of di-mercapto and tri-thioderivatives according to the present invention, as corrosion inhibitoradditives in paint formulations. Specifically, the application oftrithiocyanuric acid-di-cyclohexylamine, in a salt of a 1:1 ratio, as anadditive in a typical coil primer formulation, is disclosed.

The coil primer formulation prepared was identical to the testformulation described in Referential Example 4 (See Table D), exceptthat the corrosion inhibitor constituent consisted of 120 parts byweight Sr-doped amorphous silica, prepared according to Example 13, and30 parts by weight of trithiocyanuric acid-di-cyclohexylamine, in a saltof a 1:1 ratio. This was introduced into the formulation to end up with1006.8 parts by weight of paint and ground to 6.5-7.0 fineness on theHegman. The trithiocyanuric acid-di-cyclohexylamine 1:1 salt wassynthesized according to Referential Example 5 of the present invention.

Consequently, the corrosion inhibitor constituent of the testformulation according to Example 19 consists of an ordinary physicalmixture of the above two components. The results are shown in Table 18(See Example 20).

Example 20

This examples demonstrates the efficiency of di-mercapto, derivatives,in general, and of trithiocyanuric acid and its derivatives, inparticular, as corrosion inhibitor pigments or additives in coil primerformulations and on typical coil substrates, such as galvanized steel.It will be, however, apparent to one skilled in the art that the conceptof the present invention applies for primers intended for steelprotection in general.

In order to comparatively assess the corrosion inhibitor activity oftrithiocyanuric acid and its derivatives, the test primers ofReferential Example 4 & Example 19, along with control formulations A &B from Referential Example 4, were applied by wire-wound rod, on severalgalvanized steel panels (from L.T.V. Steel Co.), at 0.6-0.7 mil dry filmthickness, aged for at least 2 (two) days at room temperature, scribedand subsequently subjected to salt spray exposure (according to ASTMB-117).

The scribes were applied in the typical cross form, and, in order to cutthrough the protective galvanic zinc coating from the area of thescribes, at appropriate depth. During salt spray exposure, the coatings'physical state was assessed periodically by visual examination. Scribeareas were observed for the absence or presence of corrosion products(white rust), and “field” areas were observed for the physical integrityof coatings and the presence of white rust.

Notably, the protective performance of the tested coatings was qualifiedby the service life of coatings, defined as the total hours of saltspray exposure that result in extensive corrosion along the scribes andconsiderable corrosion in the “field” areas. Service life of a coatingis considered directly proportional to the related pigments' oradditives' corrosion inhibitor performance, which is convenientlyqualified by E_(i), the Inhibitor Efficiency Index, defined as:

E _(i)=100[(service life)_(TEST)−(service life)_(CONTROL)]/(servicelife)_(CONTROL).

It is important to note, that the service life of control formulation Afrom Referential Example 4, containing SrCrO₄ as a corrosion inhibitorpigment, was considered as the test control, or (servicelife)_(CONTROL).

It will be apparent, that values of E_(i)>0 indicate comparativelybetter corrosion inhibitor performance than the control's (SrCrO₄'s)performance, whereas values of E_(i)<0 indicate a poorer corrosioninhibitor performance than that of the control. The test results aresummarized below in table 18.

TABLE 18 Inhibitor Pigment or Service life of Test additive/coatingCoating (hours) E_(i) % 1. Trithiocyanuric acid-di- 3000 87cyclohexylamine, 1:1 salt and Sr-doped amorphous silica mixture, asdescribed by the test primer in Example 19. 2. Trithiocyanuric acid +Sr- 2000 25 doped amorphous silica pigment grade mixture, as describedby the test primer in table D (Ref. Ex. 4). 3. SrCrO₄, as described by1600 0 control A in table D (Ref. Ex. 4) 4. Sr-doped amorphous silica,1000 −37 as described by control A in table D (Ref. Ex. 4).

The disclosed E_(i) values indicate that, in comparison with Sr-dopedamorphous silica, trithiocyanuric acid and trithiocyanuricacid-di-cyclohexylamine, 1:1 salt significantly extend the service lifeof the coatings. Trithiocyanuric acid extends the service life of coilcoatings on galvanized steel by 100% over Sr-doped amorphous silica, andtrithiocyanuric acid-di-cyclohexylamine, 1:1 salt, extends the servicelife by 200% over Sr-doped amorphous silica. Likewise, both compoundsdisplayed considerably better corrosion inhibitor performance thanSrCrO₄, and more specifically trithiocyanuric acid-di-cyclohexylamine,1:1 salt displayed the best corrosion inhibiting performance. Also,Sr-doped amorphous silica, as expected, displayed significantly poorerinhibitor performance than SrCrO₄.

Example 21

This example is intended to demonstrate one possible application of apigment grade Htlc-DMTD derivative, specifically in a typical coilcoating paint formulation.

The pigment grade Htlc-DMTD derivative (at 22% DMTD content) wassynthesized according to the process described in Example 4, and thederivative was formulated into a solvent-borne, polyester-based coilformulation as described in Referential Example 4. (See ControlFormulation A, in Table D). In this formulation, 143.5 g of pigmentgrade SrCrO₄ was replaced with 92.0 g of Htlc-DMTD. The resultantpolyester-based coil formulation was applied by wire-wound rod, ongalvanized steel panels (from L.T.V. Steel Co.), at 0.6-0.7 mil dry filmthickness, aged for at least 2 (two) days at room temperature, scribedand subsequently subjected to protective performance test.

Example 22

This example is intended to demonstrate the application of a pigmentgrade Htlc-DMTD derivative in a typical solvent-borne, two-componentaircraft primer formulation specifically recommended for aluminumprotection.

The pigment grade Htlc-DMTD derivative (at 10% DMTD content) wassynthesized according to the process described in Example 3 and wasformulated into the aircraft primer formulation presented in Example 15(see Control Primer A in Table 15).

In this formulation, 107.5 g of pigment grade SrCrO₄ was replaced with69.0 g of Htlc-DMTD. The resulted aircraft primer was applied bywire-wound rod, on Alodine 1200 (MIL-C-5541) treated bare 2024 T-3aluminum panels (from The Q-Panel Co.), at 0.6-0.8 mils dry filmthickness, aged for 7 days at room temperature, scribed and subsequentlysubjected to protective performance test.

Example 23

This example is intended to demonstrate the usefulness, as corrosioninhibitor pigments, of double hydroxides with layered structure of[M(II)_(1-x)M(III)_(x)(OH⁻)₂] [A^(n−) _(x/n)·y H₂O] general compositionand anion-exchanged with selected anionic species. More specifically,this example demonstrates the effectiveness of the Htlc-DMTD host-guestcomposition (for example as synthesized according to Example 4) aspigment grade inhibitors of the atmospheric corrosion of Al 2024.

The inhibitor performance test on the pigment grade Htlc-DMTD derivativewas carried out according to the procedure developed by M. Kendig and M.Hon at the Rockwell Scientific Company (see patent application Ser. No.10/690,787, “Apparatus for the Rapid Evaluation of Corrosion InhibitingActivity of Paint and Coatings”).

This procedure essentially measures the rate of the cathodic oxygenreduction reaction (as related and directly proportional cathodiccurrent in the absence (I_(no inh)) and, respectively, in the presence(I_(inh)) of dissolved corrosion inhibitor species, and specifically inthe present example, on an immersed Cu rotating disk cathode. It will beapparent that the Cu cathode, in this example, models the cathodicallypolarized Cu-rich intermetallics sites on Cu-rich Al alloy surfaces andallows the assessment of inhibitors dissolved, dispersed or leached into(from coatings) the aqueous immersion phase, with latter typically beinga neutral 5% NaCl solution.

The effectiveness of inhibitors is quantified by the InhibitorStrength=I_(no inh)/I_(inh) or by

R, the Reversibility Factor=I_(r)/I_(no inh), where I_(r) is measured inthe absence of an inhibitor on an electrode previously exposed toinhibitor solution.

It will be apparent that high Inhibitor Strength and low ReversibilityFactor values indicate high inhibitor efficiency and vice versa.

The experimental results on Htlc-CO₃ (control), Htlc-DMTD at 13% loadand at 20% load (with the latter being synthesized according to theprocess stated in Example 4), all dispersed in 5% neutral NaCl, as wellas on CrO₄ ⁽²⁻⁾(standard) dissolved at 10 mM concentration and pH=6, arepresented below in Table 19.

TABLE 19 Inhibitor Reversibility Inhibitor Strength Factor, % Htlc-CO₃1.2 93 CrO₄ ⁽²⁻⁾, 10 mM, pH = 6 62.5 27 Htlc-DMTD, 13% load 74.2 3Htlc-DMTD, 20% load 94.1 2 (Example 4)

The presented experimental data indicates that the Htlc-DMTD derivativesfunction as a reservoir of inhibitor species, it operates by releasingDMTD guest species into aqueous medium by an anion exchange mechanism.Notably, the inhibitor efficiency of Htlc-DMTD derivative appears to bedirectly proportional to the load of the DMTD guest species. Also, theexperimental data constitute compelling evidence on the efficiency andthe corrosion inhibitor mechanism of DMTD on Cu rich Aluminum alloys,that is, on the inhibition by DMTD of the O₂ reduction process whichtakes place on discrete Cu rich intermetallics sites such as typical forAl 2024.

The foregoing is considered as illustrative only of the principles ofthe invention. Furthermore, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the invention to the exact construction and operation shown anddescribed. While the preferred embodiment has been described, thedetails may be changed without departing from the invention, which isdefined by the claims.

1. A pigment grade corrosion inhibitor comprising: a host speciescomprised of an inorganic compound, said compound having a layeredstructure; and a guest species, said guest species comprising an anionicspecies of a weak acid.
 2. The corrosion inhibitor according to claim 1,wherein said host species comprises a double hydroxide compound having astructure of:[M(II)_(1-x)M(III)_(x)(OH⁻)₂][A^(n−) _(x/n) ·yH₂O] wherein M(II) is amonovalent or divalent metal cationic species, M(III) is a trivalentmetal cationic species, and A^(n−) is said anionic species.
 3. Thecorrosion inhibitor according to claim 2, wherein said divalent ormonovalent metallic cationic species and said trivalent cationic speciesbeing present in said inhibitor in a ratio of:0.2≦M(III)/(M(II)+M(III))≦0.4.
 4. The corrosion inhibitor according toclaim 2, wherein said anionic species being present in said inhibitor ina range of:1/n≦A^(n−)/M(III)≦1.
 5. The corrosion inhibitor according to claim 2,wherein said divalent or monovalent metallic cationic species isselected from the group consisting of: Mg(II), Cu(II), Ni(II), Co(II),Zn(II), Fe(II), Mn(II), Cd(II), Pb(II), Ca(II), Sr(II), Li(I), andvarious combinations of all of the above; and said trivalent metalliccationic species is selected from the group consisting of: Al(III),Ga(III), Ni(III), Co(III), Fe(III), Mn(III), Cr(III), V(III), Ti, In,Ce(III), La(III), or combinations of all of the above.
 6. The corrosioninhibitor according to claim 5, wherein said guest species is selectedfrom the group consisting of: ortho-phosphoric, pyrophosphoric,tripoly-phosphoric, polyphosphoric acid; mono- and di-alkyl oraryl-esters of ortho-phosphoric and pyro-phosphoric acid;metaphosphoric, trimeta-phosphoric, poly-metaphosphoric acid;phosphorous (phosphonic) acid; derivatives of phosphonic acid; alkyl andaryl esters of thio-phosphoric and dithio-phosphoric acid; molybdic,phospho-molybdic, silico-molybdic acid; boric acid; cyanamidic acid;nitrous acid; derivatives of thio- and dithiocarbonic acid, includingo-alkyl esters; derivatives of dithiocarbamic acid, including N-alkyldithiocarbamates; pyrrolidinecarbodithioic acid; thio-organic compoundsfunctionalized with at least one —SH group of acidic character,including: 2,5-dimercapto-1,3,4-thiadiazole (DMTD),2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4 thiadiazole, trithiocyanuricacid (TMT), and dithiocyanuric acid, various N-, S- and N,N-, S,S- andN,S-substituted derivatives of the above DMTD and TMT compounds; variousS-substituted derivatives of trithiocyanuric acid; dimer and polymerderivatives of the above DMTD and TMT compounds, including 5,5′dithio-bis(1,3,4 thiadiazole-2(3H)-thione or (DMTD)₂, and (DMTD)_(n),polymers of DMTD and (TMT)₂, dimers and polymers of TMT; variouscombinations of all the above; soluble salts of DMTD and TMT;poly-ammonium salt of DMTD or (DMTD)_(n) and TMT formed with polyamines;mercapto-benzothiazole, mercapto-benzoxazole, mercapto-benzimidazole,and combinations, thereof; di- and poly-mercapto compounds, including:di-mercapto derivatives of thiophene, pyrrole, furane, diazoles, andthiadiazoles; di- and tri-mercapto derivatives of pyridine, diazines,triazines, benzimidazole, and benzothiazole, includingdimercaptopyridine, 2,4-dithiohydantoine, and2,4,-dimercapto-6-amino-5-triazine; and carboxylic and di-carboxylicacids, including ascorbic, salicylic acid, phthalic acid, nitro-phthalicacid, succinic acid, and derivatives of succinic acid, including1-(benzothiazol-2-ylthio)succinic acid.
 7. The corrosion inhibitoraccording to claim 2, wherein the host species comprises at least onemetallic cation selected from the group consisting of: Mg, Ca, Sr, La,Ce, Zn, Fe, Al, Bi, and various combinations of all of the above.
 8. Thecorrosion inhibitor according to claim 7, wherein the guest speciescomprises at least one anion selected from the group consisting of: MoO₄⁻, PO₄ ⁻, HPO₃ ⁻, polyphosphates, BO₂ ⁻, SiO₄ ⁻, NCN⁻, WO₂ ⁻, andvarious combinations of all of the above.
 9. The corrosion inhibitoraccording to claim 2, wherein said host species comprises a hydrotalcitecompound.
 10. The corrosion inhibitor according to claim 9, wherein saidguest species is selected from the group consisting of:ortho-phosphoric, pyrophosphoric, tripoly-phosphoric, polyphosphoricacid; mono- and di-alkyl or aryl-esters of ortho-phosphoric andpyro-phosphoric acid; metaphosphoric, trimeta-phosphoric,poly-metaphosphoric acid; phosphorous (phosphonic) acid; derivatives ofphosphonic acid; alkyl and aryl esters of thio-phosphoric anddithio-phosphoric acid; molybdic, phospho-molybdic, silico-molybdicacid; boric acid; cyanamidic acid; nitrous acid; derivatives of thio-and dithiocarbonic acid, including o-alkyl esters; derivatives ofdithiocarbamic acid, including N-alkyl dithiocarbamates;pyrrolidinecarbodithioic acid; thio-organic compounds functionalizedwith at least one —SH group of acidic character, including:2,5-dimercapto-1,3,4-thiadiazole (DMTD),2,4-dimercapto-s-triazolo-[4,3-b]-1,3-4 thiadiazole, trithiocyanuricacid (TMT), and dithiocyanuric acid, various N-,S- and N,N-, S,S- andN,S-substituted derivatives of the above DMTD and TMT compounds; variousS-substituted derivatives of trithiocyanuric acid; dimer and polymerderivatives of the above DMTD and TMT compounds, including 5,5′dithio-bis(1,3,4 thiadiazole-2(3H)-thione or (DMTD)₂, and (DMTD)_(n),polymers of DMTD and (TMT)₂, dimers and polymers of TMT; variouscombinations of all the above; soluble salts of DMTD and TMT;poly-ammonium salt of DMTD or (DMTD), and TMT formed with polyamines;mercapto-benzothiazole, mercapto-benzoxazole, mercapto-benzimidazole,and combinations, thereof; di- and poly-mercapto compounds, including:di-mercapto derivatives of thiophene, pyrrole, furane, diazoles, andthiadiazoles; di- and tri-mercapto derivatives of pyridine, diazines,triazines, benzimidazole, and benzothiazole, includingdimercaptopyridine, 2,4-dithiohydantoine, and2,4,-dimercapto-6-amino-5-triazine; and carboxylic and di-carboxylicacids, including ascorbic, salicylic acid, phthalic acid, nitro-phthalicacid, succinic acid, and derivatives of succinic acid, including1-(benzothiazol-2-ylthio)succinic acid.
 11. A chemical composition thatinhibits corrosion of metal substrates, said chemical compositioncomprising: a first complexing agent comprising hydrotalcite or ahydrotalcite-like material; and a second complexing agent comprising atleast one corrosion inhibiting chemical compound; wherein the chemicalcomposition is characterized by at least a first corrosion inhibitingmechanism and a second corrosion inhibiting mechanism; wherein at leastone corrosion inhibiting chemical compound comprises at least one anodicpassivator.
 12. The composition of claim 11 wherein said chemicalcompound further comprises at least one cathodic inhibitor.
 13. Acorrosion inhibiting chemical composition comprising: a hydrotalcite orhydrotalcite-like complexing agent; and at least one organic corrosioninhibiting compound; wherein said chemical composition is formed byreacting in water; approximately 90-99.99% by total formula weighthydrotalcite or hydrotalcite-like material, and approximately 0.1-10% bytotal formula weight organic corrosion inhibiting compound.